Shared oscillator (STNO) for MRAM array write-assist in orthogonal STT-MRAM

ABSTRACT

Methods and structures useful for magnetoresistive random-access memory (MRAM) are disclosed. The MRAM device has plurality of magnetic tunnel junction (MTJ) stack having significantly improved performance of the free layers in the MTJ structures. The MRAM device utilizes a spin torque nano-oscillator (STNO), a metallic bit line and a plurality of orthogonal spin transfer magnetic tunnel junctions (OST-MTJs), each OST-MTJ comprising an in-plane polarizer, and a perpendicular MTJ.

FIELD

The present patent document relates generally to spin-transfer torquemagnetic random access memory and, more particularly, to a magnetictunnel junction stack having improved performance of the free layer inthe magnetic tunnel junction structure.

BACKGROUND

Magnetoresistive random-access memory (“MRAM”) is a non-volatile memorytechnology that stores data through magnetic storage elements. Theseelements are two ferromagnetic plates or electrodes that can hold amagnetic field and are separated by a non-magnetic material, such as anon-magnetic metal or insulator. In general, one of the plates has itsmagnetization pinned (i.e., a “reference layer”), meaning that thislayer has a higher coercivity than the other layer(s) and requires alarger magnetic field or spin-polarized current to change theorientation of its magnetization. The second plate is typically referredto as the free layer and its magnetization direction can be changed by asmaller magnetic field or spin-polarized current relative to thereference layer.

MRAM devices store information by changing the orientation of themagnetization of the free layer. In particular, based on whether thefree layer is in a parallel or anti-parallel alignment relative to thereference layer, either a “1” or a “0” can be stored in each MRAM cell.Due to the spin-polarized electron tunneling effect, the electricalresistance of the cell changes due to the orientation of the magneticfields of the two layers. The cell's resistance will be different forthe parallel and anti-parallel states and thus the cell's resistance canbe used to distinguish between a “1” and a “0”. One important feature ofMRAM devices is that they are non-volatile memory devices, since theymaintain the information even when the power is off. The two plates canbe sub-micron in lateral size and the magnetization direction can stillbe stable with respect to thermal fluctuations.

Spin transfer torque or spin transfer switching, uses spin-aligned(“polarized”) electrons to change the magnetization orientation of thefree layer in the magnetic tunnel junction (“MTJ”). In general,electrons possess a spin, a quantized number of angular momentumintrinsic to the electron. An electrical current is generallyunpolarized, i.e., it consists of 50% spin up and 50% spin downelectrons. Passing a current though a magnetic layer polarizes electronswith the spin orientation corresponding to the magnetization directionof the magnetic layer thus produces a spin-polarized current. If aspin-polarized current is passed to the magnetic region of a free layerin the MTJ device, the electrons will transfer a portion of theirspin-angular momentum to the magnetization layer to produce a torque onthe magnetization of the free layer. Thus, this spin transfer torque canswitch the magnetization of the free layer, which, in effect, writeseither a “1” or a “0” based on whether the free layer is in the parallelor anti-parallel states relative to the reference layer.

When a current is passed through a magnetic layer (e.g., a polarizer),the spin orientation of the electrons that flow out of the magneticlayer is generally aligned in the direction of the magnetization of themagnetic layer and will exert a spin-transfer torque in that direction(forming a transverse spin current) upon impinging on another magneticlayer. However, due to the conservation of angular moment for thesystem, the electrons on the opposite side of magnetic layer, those thatdo not go through the magnetic layer, generally have a spin orientationthat is aligned in the direction that is anti-parallel to themagnetization direction of the magnetic layer. The net effect of thisprocess is that the current applied to the magnetic layer undergoes spinfiltering, which creates a spin current on one side of the magneticlayer, with spins that are aligned with magnetization direction of themagnetic layer, and a reflected spin current on the other side of themagnetic layer, with spins that are anti-parallel to the magnetizationdirection of the magnetic layer. This effect occurs upon application ofa current to any magnetic layer, including an in-plane polarizationlayer or an out-of-plane reference magnetic layer. Thus, in a typicalMTJ, when switching the magnetization direction of the free layer in onedirection (e.g., from the parallel to anti-parallel state) is achievedusing spin transfer torque from the transverse spin current, switchingthe free layer in the other direction (e.g., from the anti-parallel toparallel states) would be achieved using spin transfer torque from thereflected spin current. This is typically accomplished by runningelectrical current through the MTJ in one direction when switching fromthe anti-parallel to parallel state and running the electrical currentthrough the MTJ in the other direction when switching from the parallelto anti-parallel state.

FIG. 1 illustrates a MTJ stack 100 for an MRAM device including amagnetic tunnel junction MTJ 130 and a top polarizer layer 150. Asshown, stack 100 includes one or more seed layers 110 provided at thebottom of stack 100 to initiate a desired crystalline growth in theabove-deposited layers. Furthermore, MTJ 130 is deposited on top ofSynthetic Anti-Ferromagnetic (SAF) layer 120. MTJ 130 includes referencelayer 132, which is a magnetic layer, a non-magnetic tunneling barrierlayer (i.e., the insulator) 134, and the free layer 136, which is also amagnetic layer. It should be understood that reference layer 132 isactually part of SAF layer 120, but forms one of the ferromagneticplates of MTJ 130 when the non-magnetic tunneling barrier layer 134 andfree layer 136 are formed on reference layer 132. As shown in FIG. 1,magnetic reference layer 132 has a magnetization direction perpendicularto its plane. As also seen in FIG. 1, free layer 136 also has amagnetization direction perpendicular to its plane, but its directioncan vary by 180 degrees.

The first magnetic layer 114 in the SAF layer 120 is disposed over seedlayer 110. SAF layer 120 also has an antiferromagnetic coupling layer116 disposed over the first magnetic layer 114. Furthermore, anonmagnetic spacer 140 is disposed on top of MTJ 130 and a polarizer 150is disposed on top of the nonmagnetic spacer 140. Polarizer 150 is amagnetic layer that has a magnetic direction in its plane, but isperpendicular to the magnetic direction of the reference layer 132 andfree layer 136. Polarizer 150 is provided to polarize a current ofelectrons (“spin-aligned electrons”) applied to MTJ structure 100.Polarizer 150 polarizes the current in a direction perpendicular(orthogonal) to those of the magnetizations of the free magnetic layer136 and reference magnetic layer 132. Further, one or more cappinglayers 160 can be provided on top of polarizer 150 to protect the layersbelow on MTJ stack 100. Finally, a hard mask 170 is deposited overcapping layers 160 and is provided to pattern the underlying layers ofthe MTJ structure 100, using a combination of reactive ion etch (RIE)and ion beam etching (IBE) processes.

Various mechanisms have been proposed to assist the free-layermagnetization switching in MTJ devices. One issue has been that torealize the orthogonal spin transfer effect for in-plane MTJ structures,large spin currents may be required for switching. The need for largeswitching currents may limit such device's commercial applicability. Oneway proposed to reduce switching current is to lower the magnetizationof the free layer. However, if the effective magnetization of the freelayer is lowered significantly, the orthogonal effect has to be limitedso that the free-layer does not go into precessional mode that wouldmake the end state of the free-layer magnetization un-deterministic.This defines the operation window for the in-plane OST structures. In anin-plane device, unlike that shown in FIG. 1, the magnetizationdirection of the reference layer and free layer are in the plane of thelayer. Another aspect of in-plane devices is that the thermal stabilityrequirements may limit the size of the MTJ devices to approximatelysixty nanometers or higher.

In contrast to MTJ structures with an in-plane free layer and aperpendicular polarizer, perpendicular MTJ structures, such as thoseshown in FIG. 1, are less prone to getting into a pure precessionalregime. This is due to the fact that in perpendicular MTJ structures,the direction of the demagnetization field and perpendicular anisotropycontributions are the same. In this case the precession is generally notan issue and the end-state is more deterministic. However, precessionmay be an issue with regards to read disturb, particularly when strongerread currents are used. The orthogonal polarizer acts on the free layermagnetization at the initial state, but when the precession takes hold,the fixed orthogonal polarizer 150 helps only half the cycle of thefree-layer magnetization rotation while it harms the other half of thecycle. This is demonstrated with reference to FIGS. 2-3. FIG. 2a-2bshows switching of a free layer 136 of an MTJ. As is seen, free layer136 has a magnetization direction 200 perpendicular to that of thepolarizer 150. The magnetization direction 200 of the free layer 136 canrotate by 180 degrees. FIGS. 2a-2b show precession about the axis of themagnetization vector of free layer 136. During precession, magneticvector 200 begins to rotate about its axis in a cone-like manner suchthat its magnetization vector 200′ deflects from the perpendicular axis202 of free layer 136. Whereas prior to initiating precession, nocomponent of magnetic vector 200 is in the plane of free layer 136, onceprecession starts, a component of magnetic vector 200′ can be found bothin-plane and orthogonal to free layer 136. As magnetic vector 200′continues to precess (i.e., to switch), the rotation of vector 200′extends further from the center of free layer 136, as is seen in FIG. 2b.

In most prior MTJ devices using a polarizer such as polarizer 150, themagnetization direction of polarizer 150 is fixed, which is shown inFIGS. 1 and 3. See also U.S. Pat. No. 6,532,164, which states that thedirection of the magnetization of the polarizing layer cannot vary inthe presence of current. Prior to current passing through the MTJ, thefree layer 136 has a magnetization direction 200 perpendicular to thatof the polarizer 150. While the magnetization direction 200 of the freelayer 136 can rotate by 180 degrees, such rotation is normally precludedby the free layer's inherent damping ability 205, which is representedby a vector 205 pointing to axis 202 (shown as a dashed line in FIG. 2aas well as FIG. 3). Axis 202 is perpendicular to the plane of free layer136. This damping 205 has value, defined by the damping constant, whichmaintains the magnetization direction of the free layer 136.

The precession of the magnetization vector during switching of the freelayer can be assisted by spin transfer torque exerted by the electronsof a spin-polarized current, which is generated in part by theorthogonal polarizer 150. Applying a current to the MTJ device 100produces a spin-polarized current, which exerts a spin transfer torqueon the magnetic vector 200. This spin transfer torque has an in-planecomponent of the spin transfer torque 210, which pushes magnetizationvector 200′ in the direction of the magnetic vector of polarizer 150throughout precession of magnetic vector 200′. In addition to thein-plane spin transfer torque 210 from the polarizer, the perpendicularspin transfer torque (not shown), generated by reference layer 132,pulls the magnetic vector 200′ towards the direction antiparallel to itsinitial direction 200, thereby causing switching of the free layer 136.In devices like those shown in FIG. 1, when the spin transfer torque 210begins to help overcome the damping 205 inherent to the free layer 136,the magnetic direction 200′ begins to precess about its axis, as shownin FIG. 2a . As seen in FIG. 3, in-plane spin transfer torque 210 helpsthe magnetization direction of the free layer 136 to precess in acone-like manner around an axis 202 perpendicular to the plane of thelayers. When a spin polarized current traverses the stack 100, themagnetization of the free layer 136 precesses in a continuous manner(i.e., it turns on itself in a continuous manner as shown in FIG. 3)with maintained oscillations until the magnetic direction of free layer136 is opposite the magnetic direction prior to the spin torque causingprecession, i.e., the magnetic direction of free layer 136 switches by180 degrees.

FIG. 3 illustrates precession of a free layer 136 of an MTJ assisted byorthogonal spin polarized current. The spin polarized electrons frompolarizer 150 provide a spin transfer torque which has a component 210in the plane of the precession (i.e., in-plane spin transfer torque)that helps overcome the damping 205 in the first half of the precession215 because the in-plane spin transfer torque 210 provided by the spinpolarized current is opposite that of the inherent damping 205 of thefree layer 136. This is shown on the right-hand side of the middleportion of FIG. 3, which illustrates the projection of spin transfertorque 210 onto the precession plane (i.e., the plane defined by axis200 and magnetization vector 200′ as it steadily precesses around axis200). However, the in-plane spin transfer torque actually harms theswitching process during the second half of the precession 220. Thereason for this is that the spin of the electrons in the spin polarizedcurrent only apply an in-plane spin transfer torque 210 in the directionof their polarization, which is aligned with the magnetic direction ofthe in-plane polarization layer 150. Thus, when the magnetic vector isin the half of the precession cycle 220 that is opposite the spin of thepolarized electrons, the in-plane spin transfer torque 210 actuallyworks with the inherent damping 205 of free layer 136 to make rotationmore difficult. This is shown in the left-hand side of the middleportion of FIG. 3 and can be seen in the projection of the spin transfertorque 210 onto the precessional plane of the free layer 136, which isdepicted on the bottom of FIG. 3. Indeed, it is the perpendicular spintransfer torque created by the reference layer 132 (not shown in FIG. 3)that overcomes the damping 205 of free layer 136 as well as the in-planespin transfer torque 210 during the half of a precession cycle where thespin of the electrons harms precession, and thus it is the referencelayer 132 that allows for completion of precession. The precessionaldynamics and the directionality of the spin transfer torque depicted inFIG. 3 are merely approximations at small magnetization polar angles anddo not necessarily reflect the phenomena occurring at largermagnetization polar angles. However, the precessional dynamics thatoccur when the magnetization vector of the free layer 132 is at smallmagnetization polar angles are, to a large extent, determinative of theefficiency of the switching process.

One solution that has been proposed to overcome this limitation is theuse of a precessional spin current (“PSC”) magnetic layer having amagnetization vector that can freely rotate in any magnetic direction,shown in FIG. 4a-b . The free layer 336 is similar to the free layer 136previously discussed, in that it has an inherent damping characteristic205 that can be overcome with the assistance of spin transfer torque.However, the polarizing layer 150 is replaced with a precessionalmagnetic layer 350. As seen in FIG. 4a , which shows the projection ontothe precessional plane of the direction of the spin transfer torque 211created by spin current passing through free layer 336, the direction ofspin transfer torque 211 changes with the rotation of PSC magnetic layer350. As seen on the right side of FIG. 4a , spin transfer torque 211causes the magnetization direction 200′ of the free layer 336 to precessin a cone-like manner around an axis 202 perpendicular to the plane ofthe layers. The PSC layer 350 and the free-layer 336 are magneticallyand/or electronically coupled such that the magnetization direction ofthe magnetization vector 270 of the PSC magnetic layer 350 follows theprecessional rotation of the magnetic vector of the free layer 336.

As seen in on the right-hand side of FIG. 4a , the spin polarizedelectrons provide torque 211 that helps to overcome the damping 205 inthe first half of the precession 215 because the torque 211 provided bythe spin polarized current is opposite that of the inherent damping 205of the free layer 336. In addition, torque 211 helps to overcome thedamping 205 in the second half of the precession 220 by the samemechanism. Thus, unlike prior devices having a fixed polarizationmagnetic layer 150, the spin of the electrons in the spin-polarizedcurrent applies a torque 211 in both halves of the precession cycle,including the half of the precession cycle 220 where devices with fixedpolarization magnetic layers 150 actually harmed precession. As is seen,the torque 211 continues to help overcome the inherent damping 205 offree layer 136 throughout the entire precession cycle. An MRAM deviceutilizing an MTJ structure with a PSC is depicted in FIG. 5.

However, because of the chirality of perpendicular MTJ structures thatutilize a PSC, such as the structure shown in FIG. 5, the PSC onlyenhances switching of the free layer in one direction (i.e., from theparallel state to the anti-parallel state, FIG. 4a ), but not the other(i.e., from the antiparallel state to the parallel state, FIG. 4b ). Asdiscussed above, when switching the free layer 336 from the firstdirection to the second direction, the spin current is generated by theelectrons passing through the PSC layer and the in-plane spin transfertorque 211 is in line with the magnetic vector of the PSC layer (FIG. 4a). However, during switching free layer 336 from the second direction tothe first direction, it is the reflected spin current from PSC layerthat imparts the in-plane spin transfer torque 211′ on the free layer336. As shown in FIG. 4b , the in-plane spin transfer torque 211′ causedby the reflected spin current is in the direction anti-parallel to themagnetic vector 270 of the PSC layer 350. When the magnetic vector 270is aligned with the magnetic vector 200, the in-plane spin transfertorque 211′ might actually enhance the damping characteristic 205 of thefree layer 336. Therefore, when the precession of the magnetic vector270 of the PSC layer 350 is synchronized with the precession of themagnetic vector 200 of the free layer 336, the in-plane spin transfertorque 211′ might enhance the damping characteristic 205 throughout theentire precession 220′. Thus, the PSC layer can be highly effective atincreasing the switching efficiency of the free layer in one direction,but may actually hamper switching in the other direction.

Thus, in prior devices that utilize in-plane polarization layers havinga fixed magnetization direction and having a free magnetic layer 150that is perpendicular to the plane of the device, once the precessionholds, the in-plane spin transfer torque has no net positive effect onthe switching mechanism for a full three hundred sixty degreeprecession. Moreover, in prior devices that utilize a PSC magneticlayer, the in-plane spin transfer torque enhances the switching of thefree layer throughout the precession from the first direction to thesecond direction, but might not enhance the switching of the free layerfrom the second direction to the first direction.

Therefore, there is a need for a spin torque transfer device thatreduces the amount of current needed for switching from bothmagnetization directions while also switching at high speeds andrequiring reduced chip area.

SUMMARY

An MRAM device is disclosed that has a magnetic tunnel junction stackhaving a significantly improved performance of the free layer in themagnetic tunnel junction structure that requires significantly lowerswitching currents and which significantly reduces switching times forMRAM applications and maintains this characteristic for both switchingdirections (AP to P and P to AP)

In one embodiment, a magnetic device includes a plurality of orthogonalspin transfer magnetic tunnel junctions (OST-MTJs) in a first plane.Each OST-MTJ comprises a polarization magnetic layer, a non-magneticspacer, a reference magnetic layer, a non-magnetic tunnel barrier layer,and a free magnetic layer. The in-plane polarization magnetic layer isseparated from the free magnetic layer by the non-magnetic spacer. Thefree magnetic layer is separated from the reference magnetic layer bythe non-magnetic tunnel barrier layer. The in-plane polarizationmagnetic layer has a magnetization vector that is parallel to the firstplane. The reference magnetic layer has a magnetization vector that isperpendicular to the first plane and has a fixed magnetizationdirection. The free magnetic layer has a magnetization vector that isperpendicular to the first plane and has a magnetization direction thatcan switch from a first magnetization direction to a secondmagnetization direction and from the second magnetization direction tothe first magnetization direction. The switching process involvesprecessions at a precession radius around an axis perpendicular to thefirst plane, and the magnetization vector of the free magnetic layer hasa predetermined precession frequency. The magnetic device also includesa metallic bit line in a second plane and coupled to the plurality ofOST-MTJs. The magnetic device also includes a spin torque nanooscillator (STNO) in a third plane and coupled to the metallic bit line.The STNO comprises an in-plane spin torque oscillator layer, anon-magnetic spin torque oscillator barrier layer, and a perpendicularspin torque oscillator layer. The in-plane spin torque oscillator layeris separated from the perpendicular spin torque oscillator layer by thenon-magnetic spin torque oscillator barrier layer. The in-plane spintorque oscillator layer has a magnetization vector that precesses aroundan in-plane anisotropy axis or precesses in the third plane uponapplication of a programming voltage pulse. The perpendicular spintorque oscillator layer has a magnetization vector that precesses aroundan out-of-plane anisotropy axis upon application of the programmingvoltage pulse. Application of the programming voltage pulse to themagnetic device results in a switching current pulse. The switchingcurrent pulse alternates between a maximum current value and a minimumcurrent value at a first frequency. Application of the switching currentpulse to the in-plane polarization magnetic layer, the non-magneticspacer, and the MTJ results in a spin-polarized current havingspin-polarized electrons. The spin-polarized current alternates betweena maximum spin-current value and a minimum spin-current value at thefirst frequency. The spin-polarized electrons exert a spin transfertorque on the magnetization vector of the free magnetic layer. The spintransfer torque alternates between a maximum magnitude and a minimummagnitude at the first frequency. The first frequency is synchronizedwith the predetermined precession frequency of the free magnetic layer,thereby causing the spin transfer torque to be at the maximum magnitudewhen the spin transfer torque increases the precession radius of themagnetization vector of the free magnetic layer, and at the minimummagnitude when the spin transfer torque decreases the precession radiusof the magnetization vector of the free magnetic layer. In this way, theswitching process of the free magnetic layer is improved from the firstmagnetization direction to the second magnetization direction and fromthe second magnetization direction to the first magnetization direction.

In another embodiment, a difference in frequency between the firstfrequency and the predetermined precession frequency of the freemagnetic layer is less than twenty percent of the predeterminedprecession frequency of the free magnetic layer.

In another embodiment, a difference in frequency between the firstfrequency and the predetermined precession frequency of the freemagnetic layer is less than ten percent of the predetermined precessionfrequency of the free magnetic layer.

In another embodiment, a difference in frequency between the firstfrequency and the predetermined precession frequency of the freemagnetic layer is less than five percent of the predetermined precessionfrequency of the free magnetic layer.

In another embodiment, a difference in frequency between the firstfrequency and the predetermined precession frequency of the freemagnetic layer is less than two percent of the predetermined precessionfrequency of the free magnetic layer.

In another embodiment, the metallic bit line comprises Ruthenium orRhodium.

In another embodiment, the metallic bit line comprises a layer ofRuthenium, the layer of Ruthenium being between 2 and 10 angstromsthick.

In another embodiment, the magnetization vector of the in-plane spintorque oscillator layer and the magnetization vector of the polarizationmagnetic layer are magnetically coupled.

In another embodiment, the programming voltage pulse comprises a directvoltage.

In another embodiment, the reference magnetic layer comprises CoFeB, thenon-magnetic tunnel barrier layer comprises MgO, the free magnetic layercomprises CoFeB, the non-magnetic spacer comprises MgO, and the in-planepolarization magnetic layer comprises CoFeB.

In another embodiment, the in-plane spin torque oscillator layercomprises CoFeB and the perpendicular spin torque oscillator layercomprises CoFeB.

In another embodiment, the non-magnetic spin torque barrier layercomprises MgO.

In another embodiment, the free magnetic layer comprises CoFeB.

In another embodiment, the non-magnetic tunnel barrier layer comprisesMgO.

In another embodiment, the in-plane polarization magnetic layercomprises CoFeB, Fe, FeV, or FeB.

In another embodiment, the in-plane polarization magnetic layercomprises CoFeB.

In another embodiment, the non-magnetic spacer comprises MgO.

In another embodiment, the magnetization vector of the polarizationmagnetic layer is fixed.

In another embodiment, a magnetic device includes a plurality oforthogonal spin transfer magnetic tunnel junctions (OST-MTJs) in a firstplane. Each OST-MTJ comprises a reference magnetic layer, a non-magnetictunnel barrier layer, a free magnetic layer, a non-magnetic spacer, andan in-plane polarization magnetic layer. The non-magnetic tunnel barrierlayer is disposed over the reference magnetic layer. The referencemagnetic layer has a magnetization vector that is perpendicular to thefirst plane and has a fixed magnetization direction. The free magneticlayer is disposed over the non-magnetic tunnel barrier layer. The freemagnetic layer has a magnetization vector that is perpendicular to thefirst plane and has a magnetization direction that can switch from afirst magnetization direction to a second magnetization direction andfrom the second magnetization direction to the first magnetizationdirection. The switching process involves precessions at a precessionradius around an axis perpendicular to the first plane. Themagnetization vector of the free magnetic layer has a predeterminedprecession frequency. The non-magnetic spacer is disposed over the freemagnetic layer. The in-plane polarization magnetic layer is disposedover the non-magnetic spacer. The in-plane polarization magnetic layerhas a magnetization vector that is parallel to the first plane. Themagnetic device also includes a metallic bit line in a second plane. Themetallic bit line is disposed over the plurality of OST-MTJs. Themagnetic device also includes a spin torque nano oscillator (STNO) in athird plane and coupled to the metallic bit line. The STNO comprises anin-plane spin torque oscillator layer, a non-magnetic spin torqueoscillator barrier layer, and a perpendicular spin torque oscillatorlayer. The in-plane spin torque oscillator layer is separated from theperpendicular spin torque oscillator layer by the non-magnetic spintorque oscillator barrier layer. The in-plane spin torque oscillatorlayer has a magnetization vector that precesses around an in-planeanisotropy axis or precesses in the third plane upon application of aprogramming voltage pulse. The perpendicular spin torque oscillatorlayer has a magnetization vector that precesses around an out-of-planeanisotropy axis upon application of the programming voltage pulse.Application of the programming voltage pulse to the magnetic deviceresults in a switching current pulse. The switching current pulsealternates between a maximum current value and a minimum current valueat a first frequency. Application of the switching current pulse to thein-plane polarization magnetic layer, the non-magnetic spacer, and theMTJ results in a spin-polarized current having spin-polarized electrons.The spin-polarized current alternates between a maximum spin-currentvalue and a minimum spin-current value at the first frequency. Thespin-polarized electrons exert a spin transfer torque on themagnetization vector of the free magnetic layer. The spin transfertorque alternates between a maximum magnitude and a minimum magnitude atthe first frequency. The first frequency is synchronized with thepredetermined precession frequency of the free magnetic layer, therebycausing the spin transfer torque to be at the maximum magnitude when thespin transfer torque increases the precession radius of themagnetization vector of the free magnetic layer, and at the minimummagnitude when the spin transfer torque decreases the precession radiusof the magnetization vector of the free magnetic layer. In this way, theswitching process of the free magnetic layer is improved from the firstmagnetization direction to the second magnetization direction and fromthe second magnetization direction to the first magnetization direction.

In another embodiment, the in-plane spin torque oscillator layer isdisposed over the metallic spacer, the non-magnetic spin torqueoscillator barrier layer is disposed over the in-plane spin torqueoscillator layer, and the perpendicular spin torque oscillator layer isdisposed over the non-magnetic spin torque oscillator barrier layer.

In another embodiment, the perpendicular spin torque oscillator layer isdisposed over the non-magnetic spin torque oscillator barrier layer, thenon-magnetic spin torque oscillator barrier layer is disposed over theperpendicular spin torque oscillator layer, and the in-plane spin torqueoscillator layer is disposed over the metallic spacer.

In another embodiment, a magnetic device includes a plurality ofreference magnetic layers in a first plane. Each reference magneticlayer has a magnetization vector that is perpendicular to the firstplane and has a fixed magnetization direction. The magnetic device alsoincludes a plurality of non-magnetic tunnel barrier layers in a secondplane, each non-magnetic tunnel barrier layer disposed over onereference magnetic layer. The magnetic device also includes a pluralityof free magnetic layers in a third plane, each free magnetic layerdisposed over one non-magnetic tunnel barrier layer. Each free magneticlayer has a magnetization direction that can switch from a firstmagnetization direction to a second magnetization direction and from thesecond magnetization direction to the first magnetization direction. Theswitching process involves precessions at a precession radius around anaxis perpendicular to the third plan. The magnetization vector of thefree magnetic layer has a predetermined precession frequency. Theplurality of reference magnetic layers, the plurality of non-magnetictunnel barrier layers and the plurality of free magnetic layers form aplurality of magnetic tunnel junctions (MTJs). The magnetic device alsoincludes a plurality of non-magnetic spacers in a fourth plane, eachnon-magnetic spacer disposed over one free magnetic layer. The magneticdevice also includes a plurality of in-plane polarization magneticlayers in a fifth plane, each in-plane polarization magnetic layerdisposed over one non-magnetic spacer. The in-plane polarizationmagnetic layer has a magnetization vector that is parallel to the fifthplane. The magnetic device also includes a metallic bit line in a sixthplane and disposed over the plurality of in-plane polarization magneticlayers. The magnetic device also includes an in-plane spin torqueoscillator layer in a seventh plane and disposed over the metallic bitline. The in-plane spin torque oscillator layer has a magnetizationvector that precesses around an in-plane anisotropy axis uponapplication of a programming voltage pulse. The magnetic device alsoincludes a non-magnetic spin torque oscillator barrier layer in aneighth plane and disposed over the in-plane spin torque oscillatorlayer. The magnetic device also includes a perpendicular spin torqueoscillator layer in a ninth plane and disposed over the non-magneticspin torque oscillator barrier layer. The perpendicular spin torqueoscillator layer has a magnetization vector that precesses around anout-of-plane anisotropy axis upon application of the programming voltagepulse. The in-plane spin torque oscillator layer, the non-magnetic spintorque oscillator barrier layer, and the perpendicular spin torqueoscillator layer form a spin torque nano oscillator (STNO). Applicationof the programming voltage pulse to the magnetic device results in aswitching voltage across the in-plane polarization magnetic layer, thenon-magnetic spacer and the MTJ. The switching voltage oscillatesbetween a maximum voltage value and a minimum voltage value at a firstfrequency. The first frequency is synchronized with the predeterminedprecession frequency of the free magnetic layer, thereby enhancing theefficiency of the switching process from the first magnetizationdirection to the second magnetization direction and from the secondmagnetization direction to the first magnetization direction. In thisway, the switching process of the free magnetic layer is improved fromthe first magnetization direction to the second magnetization directionand from the second magnetization direction to the first magnetizationdirection.

In another embodiment, application of the switching voltage across thein-plane polarization magnetic layer, the non-magnetic spacer and theMTJ generates a spin-polarized current having spin-polarized electrons.The spin-polarized current alternates between a maximum spin-currentvalue and a minimum spin-current value at the first frequency. Thespin-polarized electrons exert a spin transfer torque on themagnetization vector of the free magnetic layer.

In another embodiment, the first frequency is synchronized with thepredetermined precession frequency of the free magnetic layer, therebycausing the spin transfer torque to be at a maximum magnitude when thespin transfer torque increases the precession radius of themagnetization vector of the free magnetic layer, and at a minimummagnitude when the spin transfer torque decreases the precession radiusof the magnetization vector of the free magnetic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included as part of the presentspecification, illustrate the presently preferred embodiments and,together with the general description given above and the detaileddescription given below, serve to explain and teach the principles ofthe MTJ devices described herein.

FIG. 1 illustrates a conventional perpendicular MTJ stack with anin-plane polarizer for an MRAM device.

FIGS. 2a-2b illustrates the precession of the free layer in an MTJ.

FIG. 3 illustrates the precession of the free layer in an MTJ used witha polarizing magnetic layer having a fixed magnetization direction.

FIGS. 4a-4b illustrates the precession of the free layer in an MTJ witha precessional spin current magnetic layer having a magnetizationdirection that rotates freely.

FIG. 5 illustrates an MTJ stack for an MRAM device having a precessionalspin current magnetic layer.

FIGS. 6a-6b illustrate the function of the STNO in converting theprogramming voltage pulse into a switching current pulse that oscillatesbetween two current values.

FIGS. 7a-7b illustrates the precession of the free layer in an MTJ witha polarizer magnetic layer having a fixed magnetization direction thatutilizes a programming current pulse that is generated by applying aconstant voltage across an MRAM device comprising an STNO and anorthogonal spin transfer perpendicular MTJ (OST-MTJ) structure thatcomprises an in-plane polarizing magnetic layer having a fixedmagnetization direction and a perpendicular MTJ.

FIG. 8 illustrates an MTJ stack for an MRAM device that utilizes an STNOand an OST-MTJ structure.

FIG. 9 illustrates the voltage across a magnetic device that utilizes anSTNO and an OST-MTJ structure.

FIG. 10 is a graph of several simulations illustrating the improvementin performance of MTJ devices having an OST-MTJ structure, with an ACswitching current injected from an STNO.

FIGS. 11a-11b are graphs of simulations illustrating the improvement inperformance of MTJ devices having an OST-MTJ structure, with an ACswitching current injected from an STNO.

FIG. 12 illustrates an MRAM device utilizing a STNO connected to aplurality of OST-MTJ structures by a metallic bit line, where eachOST-MTJ structure comprises an in-plane polarizing magnetic layer havinga fixed magnetization direction, and a MTJ structure.

FIG. 13 illustrates the voltage across an MRAM device utilizing a STNOconnected to a plurality of OST-MTJ structures by a metallic bit line,where each OST-MTJ structure comprises an in-plane polarizing magneticlayer having a fixed magnetization direction, and a MTJ structure.

FIG. 14 illustrates an alternative embodiment of an MRAM deviceutilizing a STNO connected to a plurality of OST-MTJ structures by ametallic bit line, where each OST-MTJ structure comprises an in-planepolarizing magnetic layer having a fixed magnetization direction, and aMTJ structure.

The figures are not necessarily drawn to scale and the elements ofsimilar structures or functions are generally represented by likereference numerals for illustrative purposes throughout the figures. Thefigures are only intended to facilitate the description of the variousembodiments described herein; the figures do not describe every aspectof the teachings disclosed herein and do not limit the scope of theclaims.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to create and use methods and magnetic devices that utilize aprogramming current comprising an alternating perturbation current toassist in the switching of a magnetization vector for a magneticsemiconductor device such as an MRAM device. Each of the features andteachings disclosed herein can be utilized separately or in conjunctionwith other features to implement the disclosed system and method.Representative examples utilizing many of these additional features andteachings, both separately and in combination, are described in furtherdetail with reference to the attached drawings. This detaileddescription is merely intended to teach a person of skill in the artfurther details for practicing preferred aspects of the presentteachings and is not intended to limit the scope of the claims.Therefore, combinations of features disclosed in the following detaileddescription may not be necessary to practice the teachings in thebroadest sense, and are instead taught merely to describe particularlyrepresentative examples of the present teachings.

In the following description, for purposes of explanation only, specificnomenclature is set forth to provide a thorough understanding of thepresent teachings. However, it will be apparent to one skilled in theart that these specific details are not required to practice the presentteachings.

The present patent document discloses a MRAM device that uses a STNO, ametallic bit line, and a plurality of orthogonal spin transferperpendicular MTJ (i.e., OST-MTJ) structures, each comprising anin-plane polarization layer having a fixed magnetization direction and aperpendicular MTJ. The MRAM device is described with reference to FIGS.6-14. In one embodiment, application of a direct voltage (i.e.,programming voltage) to the STNO generates a switching current pulsethat oscillates between a maximum current value and a minimum currentvalue at a first frequency. Application of this switching current pulseto a selected OST-MTJ structure generates a spin-polarized currenthaving spin polarized electrons that alternates between a maximumspin-current value and a minimum spin-current value at a frequencydetermined by the structure and composition of the STNO (i.e., the firstfrequency). The spin-polarized electrons exert a spin transfer torque onthe magnetization vector of the free magnetic layer of the selectedOST-MTJ stack, thereby assisting in the switching of the magnetizationdirection of that free layer. In another embodiment, application of theprogramming voltage to the magnetic device results in a switchingvoltage across the selected OST-MTJ stack. The switching voltageoscillates between a maximum voltage value and a minimum voltage valueat a first frequency. Application of the programming voltage to thein-plane polarization layer and MTJ generates a spin-polarized currenthaving spin polarized electrons that alternates between a maximumspin-current value and a minimum spin-current value at the firstfrequency. The spin-polarized electrons exert a spin transfer torque onthe magnetization vector of the free magnetic layer, thereby assistingin the switching of the magnetization direction of the free layer. Inanother embodiment, the first frequency is synchronized with thepredetermined precession frequency of the magnetization vector of thefree layer, such that the in-plane spin torque component provides a netbenefit of assisting in the precessional motion of the magnetizationvector, thereby enhancing the efficiency of the switching of the freelayer.

FIG. 6a shows a schematic for the magnetization dynamics of STNO 570.Although for purposes of illustration, the magnetization dynamics of theSTNO and the free layer of the OST-MTJ are described with reference toan MRAM device having only one OST-MTJ structure (such as that shown inFIGS. 8 and 9), these dynamics are valid approximations of themagnetization dynamics found in an MRAM device having a plurality ofOST-MTJ stacks, such as device 700 in FIG. 12. Application of theprogramming voltage to the magnetic device causes the magnetizationvectors of the in-plane spin torque oscillator layer 572 and theperpendicular spin torque oscillator layer 576 to precess around theirrespective anisotropy axes, as shown on the top of FIG. 6a . Theresistance across the STNO 570 is dependent on the relative orientationof the magnetization vectors of the in-plane spin torque oscillatorlayer 572 and the perpendicular spin torque oscillator layer 576. Forexample, when the two magnetization vectors are arranged in oppositeorientations (as depicted in the middle of FIG. 6a ), STNO 570 willpossess a relatively high resistance value, compared to the relativelylow resistance value possessed by the STNO when the two magnetizationvectors are aligned (as depicted in the bottom of FIG. 6a ). Thus, asthe magnetization vectors of the in-plane spin torque oscillator layer572 and the perpendicular spin torque oscillator layer 576 precessaround their respective axes, STNO 570 alternates between a highresistance state and a low resistance state at a first frequency.

FIG. 6b depicts the effect of these precessional dynamics on theresistance across STNO 570 and the voltage across the in-planepolarization magnetic layer, the non-magnetic spacer, and the MTJ. Asdescribed above, the precessional dynamics of the magnetization vectorsof the in-plane spin torque oscillator layer 572 and the perpendicularspin torque oscillator layer 576 cause the resistance across STNO 570 tooscillate between a high resistance state and a low resistance state(shown in the top graph of FIG. 6b ) at the first frequency. As aresult, the voltage across the metallic spacer 560, the in-planepolarization magnetic layer 550, the non-magnetic spacer 540, and theMTJ structure 530 (i.e., the switching voltage) oscillates between ahigh voltage value (corresponding to the time at which the resistanceacross the STNO 570 is at the minimum resistance value) and a lowvoltage value (corresponding to the time at which the resistance acrossthe STNO 570 is at the maximum resistance value) at the same firstfrequency. The strength of the current leaving the STNO 570 and passingthrough the metallic spacer 560, the in-plane polarization magneticlayer 550, the non-magnetic spacer 540, and the MTJ structure 530 (i.e.,the switching current) is directly proportional to the switchingvoltage. Therefore, the switching current alternates between a maximumcurrent value and a minimum current value at the first frequency. In thecase of magnetic device 700, after an OST-MTJ stack has been selectedfor writing, a switching current is generated in that selected OST-MTJstack using a similar process.

In one embodiment, the programming voltage pulse is a direct voltagewith a fixed voltage value. Upon being subjected to the alternatingresistance states of the STNO 570, the resulting current leaving theSTNO 570 and (i.e., the switching current) oscillates between a maximumcurrent value and a minimum current value. In one embodiment, thefrequency at which the oscillation occurs (i.e., the first frequency) ismatched to the predetermined precession frequency of the magnetic vectorof the free layer 536 while the precession frequency is near its maximumvalue. Thus, the switching current will oscillate between the maximumand minimum current values in a manner that is synchronized with theinitial precessions of the magnetization vector of the free layer 536(i.e., the predetermined precession frequency of the free layer 536,which is the frequency at which the magnetization vector of free layer536 precesses when the vector is nearly perpendicular to the plane).However, later in the switching process, as the precession frequency ofthe free layer decreases (when the magnetization vector of the freelayer 536 precesses near or in the plane), the predetermined precessionfrequency of the free layer 536 may fall out of phase with the firstfrequency.

As described above, when the switching current is applied to thein-plane polarization layer 550 and a perpendicular MTJ 530, aspin-polarized current is formed. As the switching current alternatesbetween a maximum current value and a minimum current value, thespin-polarized current also alternates between a maximum spin-currentvalue and a minimum spin-current value at the first frequency. Inaddition, the magnitude of the spin-transfer torque exerted on the freelayer 536 is proportional to the spin-current value. Therefore, when thespin current is at the maximum spin-current value, the magnitude of thespin-transfer torque being exerted on the free layer 536 is at themaximum magnitude. When the spin current is at the minimum spin-currentvalue, the magnitude of the spin-transfer torque being exerted on thefree layer 536 is at the minimum magnitude. Therefore, in embodimentswhere the first frequency in synchronized with the predeterminedprecession frequency of the magnetization vector of the free layer 536,the magnitude of the spin transfer torque will oscillate between themaximum magnitude and minimum magnitude at a frequency that issynchronized with the precession of the free layer 536 (i.e., the firstfrequency).

FIGS. 7a and 7b show the concept behind devices that utilize a STNO 570,an in-plane polarization layer 550, and a MTJ structure 530. Again,similar magnetization dynamics occur when switching free layer 736during the process of writing a selected OST-MTJ stack 730′ in MRAMdevice 700 (see FIG. 12). The magnetization dynamics depicted in FIGS.7a and 7b are approximations of the dynamics that occur during theinitial stages of switching the magnetization vector of free layer 536(i.e., when the polar angle between axis 200 and magnetic vector 200′ issmall). At larger magnetization polar angles (i.e., more than 10 degreesfrom equilibrium), the dynamics presented in these figures no longerprovide an accurate depiction of the magnetization dynamics of the freelayer. Nevertheless, the precessional dynamics that occur at lowmagnetization polar angles are to a great extent determinative of theswitching process and are therefore useful in understanding thedisclosures herein.

Like the in-plane polarizer 150 previously discussed, the in-planepolarization layer 550 in this embodiment has a magnetic vector with afixed magnetization direction (top of FIG. 7a ). The free layer 536 inthis embodiment is similar to the free layer 136 previously discussed,in that it has an inherent damping characteristic 205 that can beovercome with the assistance of spin transfer torque. As seen in themiddle of FIG. 7a , the in-plane spin transfer torque 610 causes themagnetization direction 200′ of the free layer 536 to precess in acone-like manner around an axis 202 perpendicular to the plane of thelayers. FIG. 7a shows a progression of rotation of the magneticdirection 200′ about axis 202. As discussed, when a spin polarizedcurrent traverses the device, the magnetization of the free layer 536precesses in a continuous manner (i.e., it turns on itself in acontinuous manner as shown in the middle of FIG. 7a ) with maintainedoscillations until the magnetic direction of free layer 536 is oppositethe magnetic direction prior to the spin torque causing precession,i.e., the magnetic direction of free layer 136 switches by 180 degrees.

The spin-polarized electrons of the spin-polarized current exert a spintransfer torque on the magnetization layer of the free layer. This spintransfer torque has both an in-plane spin torque component 610 and aperpendicular spin torque component (not shown in FIG. 7a ). Theperpendicular spin torque exerts a force on the magnetization vector ofthe free layer that pulls the magnetization vector from oneperpendicular position to the other perpendicular position (e.g., fromthe parallel to the anti-parallel state). This perpendicular spin torquecomponent is caused by spin-polarization of the electrons by thereference magnetic layer 532 (depicted in FIG. 8). The in-plane spintorque 610 assists in the switching of the free layer by providing aforce that pushes the magnetization vector away from the perpendicularaxis 202, allowing the perpendicular spin transfer torque to act on themagnetization vector, thereby switching the free layer. This in-planespin torque 610 is caused by spin-polarization of the electrons by thein-plane polarization magnetic layer 550.

The in-plane spin torque 610 also enhances the precessional motion ofthe magnetization vector of the free layer. As seen on the right-handside of FIG. 7a , the spin-polarized electrons provide in-plane spintorque 610 that helps to overcome the damping 205 in the first half ofthe precession 215 because the in-plane spin torque 610 provided by thespin-polarized current is opposite that of the inherent damping 205 ofthe free layer 536. During the first half of the precession, thespin-polarized current is at or near the maximum current value, therebyimparting in-plane spin transfer torque 610 at or near the maximumspin-torque magnitude (depicted by the longer arrow). As previouslydiscussed, during the second half of the precession 215, this in-planespin torque 610′ is actually aligned with the inherent damping of thefree layer 536. However, in one embodiment, during the second half ofthe precession, the spin-polarized current is at or near the minimumcurrent value, thereby imparting in-plane spin transfer torque 610′ ator near the minimum spin-torque magnitude (depicted by the shorterarrow). The frequency at which the switching current alternates betweenits maximum current value and its minimum current value is set at afirst frequency, which is defined by the composition and structure ofthe STNO 570. The first frequency is set to be synchronized with thepredetermined precession frequency of the free layer. Therefore, themagnitude of the in-plane spin transfer torque is maximized (FIG. 7a ,610) when it enhances the precessional motion of the free layer and isminimized (FIG. 7a , 610′) when the in-plane spin transfer torqueopposes the precessional motion (bottom of FIG. 7a ). Thus, the in-planespin torque component provides a net benefit of assisting in theprecessional motion of the magnetization vector, thereby enhancing theefficiency of the switching of the free layer.

Moreover, as shown in FIG. 7b , the use of a magnetic device 500 thatcomprises STNO 570, in-plane polarization magnetic layer 550 and MTJstructure 530 can provide a net enhancement to the precession throughoutthe switching process of the free layer 536 from both the firstmagnetization direction and the second magnetization direction (i.e.,switching of the free layer from both the parallel direction to theantiparallel direction and from the antiparallel direction to theparallel direction). During switching of the free layer 536 from thesecond magnetic direction to the first magnetic direction, it is thereflected spin current generated by the in-plane polarizer that exertsthe in-plane spin transfer torque 611 on the free layer 536. Asdiscussed above, the direction of the in-plane spin transfer torque 611exerted by the reflected spin current is antiparallel to the magneticdirection of the in-plane polarizer 550 (top of FIG. 7b ). Nevertheless,the first frequency can be synchronized with the predeterminedprecession frequency of the magnetization vector 200′ of the freemagnetic layer in such a way as to maximize the in-plane spin transfertorque 611 when it opposes the damping force 205 and minimize thein-plane spin transfer torque 611′ when it enhances the damping force205 (middle of FIG. 7b ). Thus, synchronization of the oscillationfrequency of the STNO (i.e., the first frequency) with the precessionfrequency of the free layer can be accomplished regardless of thedirection of the current, achieving a net enhancement of theprecessional motion during switching of free layer 536 in bothdirections. Similar advantages can be obtained using an MRAM device,such as device 700 in FIG. 12, that includes a plurality of OST-MTJstacks connected to an STNO by a metallic bit line.

FIG. 8 shows a memory cell 500 with a OST-MTJ (comprising an in-planepolarization layer 550, a MTJ structure 530), and a STNO structure 570The OST-MTJ structure comprises at least a reference layer 532, atunneling barrier layer 540, a free layer 536, a non-magnetic spacer540, and an in-plane polarization layer 550. As shown in FIG. 8, thereference layer 532 of MTJ 530 has a magnetization direction that ispreferably perpendicular to its plane, although variations of a severaldegrees are within the scope of what is considered perpendicular. Asalso seen in FIG. 8, free layer 536 also has a magnetization vector thatis preferably perpendicular to its plane, but its direction can vary by180 degrees. A nonmagnetic spacer 540 is disposed over free layer 536.In-plane polarization magnetic layer 550 is disposed over nonmagneticspacer 540. In-plane polarization magnetic layer 550 has a magnetizationvector having a magnetic direction parallel to its plane. In-planepolarization magnetic layer 550 has a magnetization direction that ispreferably parallel to its plane, although variations of a severaldegrees are within the scope of what is considered parallel. Metallicspacer 560 is disposed over the OST-MTJ structure. STNO 570 is depositedover metallic spacer 560. STNO 570 includes in-plane spin torqueoscillator layer 572, a non-magnetic spin torque oscillator barrierlayer 574, and a perpendicular spin torque oscillator layer 576. Asshown in FIG. 12, the magnetization vector of in-plane spin torqueoscillator layer 572, has a magnetization direction that precessesaround an anisotropy axis that is parallel to the plane, althoughvariations of a several degrees are within the scope of what isconsidered parallel. In some embodiments, the magnetic direction of thein-plane spin torque oscillator layer precesses in the plane if thein-plane anisotropy is weak. As also seen in FIG. 8, the perpendicularspin torque oscillator layer 576 has a magnetization vectormagnetization direction that precesses around an anisotropy axis that isperpendicular to the plane, although variations of a several degrees arewithin the scope of what is considered perpendicular. In one embodiment,voltage source 595 generates a direct voltage.

FIG. 9 depicts the voltage across the magnetic device 500, whichcomprises STNO 570, in-plane polarization layer 550, and MTJ structure530. As the programming current pulse flows through device 500, theprogramming current pulse initiates the precessional dynamics of themagnetic vectors of the STNO 570. As discussed above, this causes theresistance across STNO 570 to oscillate between a high resistance stateand a low resistance state at a first frequency. In one embodiment, thevoltage across the entire device (labeled a V-Pulse) is fixed andconstant. V-STNO oscillates between a maximum voltage value and aminimum voltage value at the first frequency due to the oscillatingresistance value of STNO 770. Metallic spacer 560 is comprised of ahighly-conductive material, such as a metallic material. Therefore, theoutput voltage from STNO 570 is essentially equivalent to the inputvoltage for the V-pOST. Accordingly, V-pOST also oscillates between amaximum voltage value and a minimum voltage value at the firstfrequency, thereby causing the current flowing through the OST-MTJstructure (i.e., the switching current) to alternate between a maximumcurrent value and a minimum current value at the first frequency.

The results of several simulations that model the structures describedherein are seen in FIG. 10. The X axis of the graph is the voltage ofthe programming pulse that was applied to the device. The Y axis is thewrite error rate (WER), which is indicative of the how easily themagnetization vector of the free layer switches from the firstmagnetization direction to the second magnetization direction or fromthe second magnetization direction to the first magnetization direction;the lower the WER, the more effective the applied current is atswitching the free layer. The graph depicts two series of transitions:(1) switching the free layer from the parallel direction to theantiparallel direction (filled data points, indicated by “2AP” on thelegend); and (2) switching the free layer from the antiparalleldirection to the parallel direction (unfilled data points, indicated by“2P” on the legends.) To simulate a device comprising an STNO, anin-plane polarizer, and an MTJ, a switching current was used thatcomprised a direct current with a fixed value and an alternatingperturbation current superimposed on the direct current. During thewriting process of magnetic device 700 that comprises a STNO 770, ametallic bit line 760 and a plurality of OST-MTJ structures 730′, asimilar process occurs. Thus, these results demonstrate the significantimprovement provided by the various embodiments described herein. Withineach series, different sets of simulations were run, with theperturbation amplitude of the alternating current frequency set to 0%,10%, 20%, and 30% of the direct current voltage (depicted on the legendas “polarized”; “A=0.1”; “A=0.2”; and “A=0.3”, respectively). In each ofthe simulations, the alternating perturbation frequency (ω) issynchronized with the predetermined precession frequency of the freelayer (ω_(p) ₁ ) and the alternating perturbation current is applied tothe free layer for a pulse length of 3 ns.

For the first series of simulations (switching from the parallel toanti-parallel directions), both the constant non-polarized current (line10-A with filled squares) and the constant polarized current (line 10-Bwith filled circles) had a similar effect on the switching of the freelayer. The data show that application of an alternating perturbationcurrent causes a reduction in switching Voltage values for the system,with the switching Voltage values decreasing as the perturbationamplitude increases. This effect can be seen by comparing the lines forthe currents with 0% perturbation amplitude (line 10-B with filledcircles); 10% perturbation amplitude (line 10-C with filled diamonds),20% perturbation amplitude (line 10-D with filled crosses); and 30%perturbation amplitude (line 10-E with filled pentagons). The lines thatcorrespond to higher values of the perturbation amplitude lie below thelines with lower values of the perturbation amplitude, suggesting thatincreasing the perturbation amplitudes reduces the WER, i.e., it reducesthe probability that a bit is written incorrectly.

Similar results were obtained in the second series of experiments(switching from the anti-parallel to the parallel direction). Nosignificant change in switching voltage was observed between theconstant non-polarized current (line 10-F with empty squares) and theconstant polarized current (line 10-G with empty circles). However, theswitching voltage values decrease as the perturbation amplitudeincreases, as can be observed by comparing the lines with 0%perturbation amplitude (line 10-G with empty circles); 10% perturbationamplitude (line 10-H with empty diamonds), 20% perturbation amplitude(line 10-I with empty crosses); and 30% perturbation amplitude (line10-J with empty pentagons), respectively. The lines that correspond tohigher perturbation amplitude lie below the lines with smallerperturbation amplitude.

Thus, the results in FIG. 10 demonstrate improved switching with the useof a magnetic device comprising a STNO, an in-plane polarizer and a MTJwhen the oscillating impedance frequency of the STNO is synchronizedwith the predetermined precession frequency of the free layer, therebydemonstrating the significant improvement provided by the variousembodiments described herein. In general, the voltage required to switchthe magnetization vector from the anti-parallel direction to theparallel direction was considerably less than that required to switchthe magnetization vector from the parallel direction to theanti-parallel direction. Moreover, while the use of an alternatingperturbation current effectively reduces the switching voltage value forswitching in both directions, this effect is more pronounced forswitching from the parallel to the antiparallel direction because thespin-polarization efficiency from the in-plane polarizing layer to thefree layer is higher in this case.

Detailed results from a subset of the simulations from FIG. 10, whichsimulate a device having the structure described herein, are seen inFIGS. 11a-11b . In FIGS. 11a-11b , the Y axis is the magnetization inthe Z axis of the magnetization vector of a free layer from −1.0 to +1.0in a simulation modeling a magnetic device such as device 500. The Xaxis shows the amount of time it takes with switch the magnetizationdirection of free layer 536 a full 180 degrees. In the simulations, theoscillation frequency of the switching current (i.e., the firstfrequency) is matched to the precessional frequency of free layer 536.The amplitude of the perturbations is 20% of the mean voltage value(i.e., the programming voltage). FIG. 11a depicts switching from theparallel to antiparallel directions at a voltage value of 0.95 volts.FIG. 11b depicts switching from the antiparallel to the paralleldirection at a voltage value of −0.8 volts.

FIG. 12 depicts a magnetic device 700 with an STNO 770 that is connectedto a plurality of OST-MTJ structures 730′ by a metallic bit line 760. Asseen in FIG. 12, each magnetic device has a plurality of OST-MTJstructures 730′. Each OST-MTJ structure 730′ includes one or more seedlayers 710 provided at the bottom of stack 730′ to initiate a desiredcrystalline growth in the above-deposited layers. Syntheticantiferromagnetic (SAF) layer 720 is disposed over seed layer 710. SAFlayer 720 is comprised of a first SAF layer 732, anti-ferromagneticcoupling layer 716 and second SAF layer 714. Second SAF layer 714 isdeposited over seed layer 710, while anti-ferromagnetic coupling layer716 is placed over second SAF layer 714. MTJ 730 is deposited overanti-ferromagnetic coupling layer 716. OST-MTJ 730 includes first SAFlayer 732, which acts as the reference layer of the MTJ, and is alsopart of SAF layer 720. A tunneling barrier layer (i.e., the insulator)734 is disposed over first SAF layer 732 while the free layer 736 isdisposed over tunneling barrier layer 734. As shown in FIG. 12, themagnetization vector of first SAF layer 732 has a magnetizationdirection that is preferably perpendicular to its plane, althoughvariations of a several degrees are within the scope of what isconsidered perpendicular. As also seen in FIG. 12, free layer 736 alsohas a magnetization vector that is preferably perpendicular to itsplane, but its direction can vary by 180 degrees. A nonmagnetic spacer740 is disposed over free layer 736. In-plane polarization magneticlayer 750 is disposed over nonmagnetic spacer 740. In one embodiment,in-plane polarization magnetic layer 750 has a magnetization vectorhaving a magnetic direction parallel to its plane, and is perpendicularto the magnetic vector of the reference layer 732 and free layer 736.In-plane polarization magnetic layer 750 has a magnetization directionthat is preferably parallel to its plane, although variations of aseveral degrees are within the scope of what is considered parallel.Each OST-MTJ structure comprises at least a reference layer 732, atunneling barrier layer 740, a free layer 736, a non-magnetic spacer740, and an in-plane polarization layer 750. Metallic bit line 760 isdisposed over the plurality of OST-MTJ structures 730′. STNO 570 isdeposited over metallic bit line 760. STNO 770 includes in-plane spintorque oscillator layer 772, is disposed over magnetic spacer 760. Anon-magnetic spin torque oscillator barrier layer 774 is disposed overin-plane spin torque oscillator layer 772 while a perpendicular spintorque oscillator layer 776 is disposed over non-magnetic spin torqueoscillator barrier layer 774. As shown in FIG. 12, the magnetizationvector of in-plane spin torque oscillator layer 772, has a magnetizationdirection that precesses around an anisotropy axis that is parallel tothe plane, although variations of a several degrees are within the scopeof what is considered parallel. In some embodiments, the magneticdirection of the in-plane spin torque oscillator layer precesses in theplane if the in-plane anisotropy is weak. As also seen in FIG. 12, theperpendicular spin torque oscillator layer 776 has a magnetizationvector magnetization direction that precesses around an anisotropy axisthat is perpendicular to the plane, although variations of a severaldegrees are within the scope of what is considered perpendicular. One ormore capping layers 780 can be provided on top of perpendicular spintorque oscillator layer 776 to protect the layers below. In oneembodiment, voltage source 785 generates a direct voltage. Each OST-MTJstack 730′ is connected to a transistor 705. Each transistor 705 can beused as a switch, specifically directing a voltage across the OST-MTJstack to which it is coupled. Thus, a specific OST-MTJ stack can beselected for writing by switching its corresponding transistor (e.g.,transistor 705′) such that the programming voltage is generated acrossthe specified OST-MTJ stack.

Seed layer 710 in the MTJ structure shown in FIG. 12 preferablycomprises Ta, TaN, Cr, Cu, CuN, Ni, Fe or alloys thereof. Second SAFlayer 714 preferably comprises either a Co/Ni, Co/Pt or Co/Pd multilayerstructure. First SAF layer 732 preferably comprises either a Co/Ni orCo/Pt multilayer structure plus a thin non-magnetic layer comprised oftantalum having a thickness of two to five Angstroms and a thin CoFeBlayer (0.5 to three nanometers). Anti-ferromagnetic coupling layer 716is preferably made from Ru having thickness in the range of three to tenAngstroms. Tunneling barrier layer 734 is preferably made of aninsulating material such as MgO, with a thickness of approximately tenAngstroms. Free layer 736 is preferably made with CoFeB deposited on topof tunneling barrier layer 734. Free layer 736 can also have layers ofFe, Co, Ni or alloys thereof and W and Ta insertion layers to promoteperpendicular anisotropy. Spacer layer 740 over MTJ 730 can be anynon-magnetic material such as 2 to 20 Angstroms of ruthenium, 2-20Angstroms of Ta, 2-20 Angstroms of TaN, 2-20 Angstroms of Cu, 2-20Angstroms of CuN, or 2-20 Angstroms MgO layer, or or 2-20 AngstromsAl₂O₃ layer.

Nonmagnetic spacer 740 has a number of properties. For example,nonmagnetic spacer 740 physically separates the free layer 736 and thein-plane polarization magnetic layer 750. Nonmagnetic spacer 740transmits spin current efficiently from the in-plane polarizationmagnetic layer 750 into the free layer 736 because it preferably has along spin diffusion length if made metallic. Nonmagnetic spacer 740 alsopromotes good microstructure and high tunneling magnetoresistance (TMR)and helps keep the damping constant of the free layer 736 low. In oneembodiment, the nonmagnetic spacer 740 comprises MgO.

The in-plane polarization magnetic layer 750 is preferably made fromCoFeB. The in-plane polarization magnetic layer 750 can also be madewith CoFeB, Fe, FeV. or FeB. It can also be made with Co, Fe, Nimagnetic layers or can be made out of their alloys. The magnetic alloyscan also have boron, tantalum, copper or other materials.

Metallic bit line 760 has a number of properties. For example, metallicspacer 760 physically separates the STNO 770 from the in-planepolarization magnetic layers 750 of the plurality of OST-MTJ structures730′. Metallic bit line 760 also connects the STNO 770 to the pluralityof OST-MTJ structures 730′, such that a voltage can be applied acrossthe STNO 770 and a selected OST-MTJ structure 730′ during the writingprocess. Preferably, metallic bit line 760 is composed of ruthenium orrhodium. Metallic bit line 760 is preferably made from Ru havingthickness in the range of two to ten Angstroms or Rh having thickness inthe range of two to ten Angstroms. In one embodiment, when metallic bitline 760 is comprised of a thin layer of Ru (e.g., 7 Angstroms), thein-plane polarization magnetic layer 750 of the OST-MTJ structureclosest to the STNO 770 and the in-plane spin torque oscillator layer772 can be AFM coupled via stray fields, thereby minimizing the strayfield that in-plane polarization magnetic layer 750 imparts on freemagnetic layer 736. This AFM coupling can be strengthened via electronicoscillatory (RKKY) coupling.

The STNO 770 can be in either of the following configurations: (1)in-plane spin torque oscillator layer 772 disposed over spin torqueoscillator barrier layer 774, which is disposed over perpendicular spintorque oscillator layer 776; or (2) perpendicular spin torque oscillatorlayer 776 disposed over spin torque oscillator barrier layer 774, whichis disposed over in-plane spin torque oscillator layer 772. In-planespin torque oscillator layer 772 can be comprised of a SAF, such as acomposite ferromagnetic metal/metallic spacer (Ru or Rd)/ferromagneticmetal structure. The in-plane spin torque oscillator layer can be pinnedby a 5-10 nm layer of PtMn Exchange bias layer. Preferably, in-planespin torque oscillator layer 772 comprises CoFeB. Spin torque oscillatorbarrier layer 774 can be any non-magnetic material such as 2 to 20Angstroms of ruthenium, 2-20 Angstroms of Ta, 2-20 Angstroms of TaN,2-20 Angstroms of Cu, 2-20 Angstroms of CuN, or 2-20 Angstroms MgOlayer, or 2-20 Angstroms Al₂O₃ layer. Spin torque oscillator barrierlayer 774 is preferably made from MgO. The perpendicular spin torqueoscillator layer 776 can comprise 6-16 Angstroms of Ta/CoFeB.Preferably, the perpendicular spin torque oscillator layer 776 comprisesCoFeB. Finally, capping layer 780 can be any material that provides goodinterface to the in-plane layer such as Ta, TaN, Ru, MgO, Cu, etc.

In some embodiments, voltage source 785 can generate a programmingvoltage pulse that comprises direct voltage pulse. In other embodiments,the strength of the bias produced by voltage source 785 can vary overtime. Transistor 705 can have any appropriate transistor structure, suchas, for example, a bipolar transistor construction. In some embodiments,transistor 705 acts as a switch, specifically directing the programmingvoltage across the OST-MTJ stack to which it is coupled. In someembodiments, transistor 705 prevents a switching current from beinggenerated in more than one OST-MTJ stack at any given time.

FIG. 13 depicts the voltage across the magnetic device 700, whichcomprises STNO 770 and a plurality of OST-MTJ structures 730′, connectedby metallic bit line 760. One of the OST-MTJ stacks is selected forwriting (e.g., OST-MTJ stack 730″) using corresponding transistor 705′.This, in turn, enables the voltage generator 785 to create a voltageacross the STNO 770, the metallic bit line 760, and the selected OST-MTJstructure 730″. In FIG. 13, the farthest OST-MTJ stack to the right,OST-MTJ structure 730″ has been selected for writing. As the programmingcurrent pulse flows through device 700, the programming current pulseinitiates the precessional dynamics of the magnetic vectors of the STNO770. As discussed above, this causes the resistance across STNO 770 tooscillate between a high resistance state and a low resistance state ata first frequency.

In one embodiment, magnetic device 700 operates as a voltage divider(when supplied with a constant voltage pulse), with a first voltageacross STNO 770 (labeled as V-STNO in FIG. 13) and a second voltageacross the selected OST-MTJ structure 730″ (labeled as V-pOST in FIG.13). In one embodiment, the voltage across the entire device (labeled aV-Pulse) is fixed and constant. V-STNO oscillates between a maximumvoltage value and a minimum voltage value at the first frequency due tothe oscillating resistance value of STNO 770. Metallic bit line 760 iscomprised of a highly-conductive material, such as a metallic material.Therefore, the output voltage from STNO 770 is essentially equivalent tothe input voltage for the V-pOST. Accordingly, V-pOST also oscillatesbetween a maximum voltage value and a minimum voltage value at the firstfrequency. Moreover, the strength of the current flowing out of STNO 770and passing through the selected OST-MTJ structure 730″ (i.e., theswitching current) is directly proportional to the voltage value ofV-pOST. Thus, the switching current alternates between a maximum currentvalue and a minimum current value at the first frequency.

The manner in which a bit is written using magnetic device 700 thatcomprises STNO 770, metallic bit line 760 and a plurality of OST-MTJstructures 730′ will now be described. First, a particular OST-MTJ stack(such as OST-MTJ stack 730″ in FIG. 13) is selected for writing. In someembodiments, this selection is enabled by the presence of transistor705, which can be selectively activated (e.g., transistor 705′ in FIG.13), thereby allowing a voltage to be applied across the selectedOST-MTJ stack 730″ to which the transistor is coupled. Next, anelectrical voltage is supplied, for example, by voltage source 785,which results in an electrical current (i.e., the programming current)through the perpendicular spin torque oscillator layer 776, thenon-magnetic spin torque barrier layer 774, the in-plane spin torqueoscillator layer 772, the magnetic bit line 760, and the various layersof the selected OST-MTJ stock 730″ (e.g., the in-plane polarizationmagnetic layer 750, the non-magnetic spacer 740, the free magnetic layer736, the non-magnetic tunneling barrier layer 734, and the referencelayer 732). Application of the programming voltage to STNO 770 causesthe magnetization vectors of the perpendicular spin torque oscillatorlayer 776 and the in-plane spin torque oscillator layer 772 to precessaround their respective axes. The precession of the magnetic vectorsperpendicular spin torque oscillator layer 776 and the in-plane spintorque oscillator layer 772 causes the resistance across the STNO tooscillate between a maximum resistance value and a minimum resistancevalue. The net effect is that the current leaving the STNO and passingthrough the in-plane polarization magnetic layer 750 of the selectedOST-MTJ stack 730″ (i.e., the switching current) alternates between amaximum current value and a minimum current value at a first frequency.

Application of the switching current to the selected OST-MTJ stack 730″(in particular, in-plane polarization layer 750, free layer 736 andreference layer 732) creates a spin polarized current that passesthrough non-magnetic spacer layer 740, free magnetic layer 736,tunneling barrier layer 734, and reference magnetic layer 732 of theselected OST-MTJ stack. The spin polarized current exerts a spintransfer torque on free magnetic layer 736, which helps overcome theinherent damping of the magnetic material making up the free layer 736.The spin transfer torque is composed of an in-plane spin transfer torqueand a perpendicular spin transfer torque. In one embodiment, whenswitching the free layer 736 in one direction (e.g., from the paralleldirection to the anti-parallel direction), the in-plane spin transfertorque is caused by the transverse spin current generated by thein-plane polarization magnetic layer 750 and the perpendicular spintransfer torque is caused by the reflected spin current generated by thereference magnetic layer 732. This causes the magnetization vector ofthe free magnetic layer 736 to precess about its axis, which is shown inFIG. 7 a.

The spin-polarized current, which is generated by application of theswitching current to the selected OST-MTJ structure 730″, alternatesbetween a maximum spin-current value and a minimum spin current value atthe same first frequency. The magnitude of the spin transfer torqueexerted on the magnetic vector of the free layer 736 is proportional tothe spin-current value because at higher spin-current values, there aremore spin-polarized electrons flowing through free layer 736. Therefore,the magnitude of the spin transfer torque alternates between a maximumspin torque magnitude and a minimum spin torque magnitude at the samefirst frequency.

The in-plane spin transfer torque causes the magnetic vector of the freemagnetic layer 736 to precess, as depicted in FIG. 7a . The precessionof the magnetic vector of free magnetic layer 736 occurs at apredetermined precession frequency. In some embodiments, the firstfrequency is synchronized with the predetermined precession frequency.Because the magnitude of the in-plane spin transfer torque alternatesbetween the maximum spin torque magnitude and the minimum spin torquemagnitude at the first frequency, these magnitude oscillations are alsosynchronized with the predetermined precession frequency. As depicted inFIG. 7a (middle, right), the in-plane spin transfer torque is at or nearthe maximum spin torque magnitude when the in-plane spin transfer torqueopposes the inherent damping of the free magnetic layer 736. Also shownin FIG. 7a . (middle left), the in-plane spin transfer torque is at ornear the minimum spin torque magnitude when the in-plane spin transfertorque enhances the inherent damping of the free magnetic layer 736.Therefore, the in-plane spin transfer torque from in-plane polarizer 750provides a net spin torque throughout the precession that opposes thedamping characteristic of the free magnetic layer 736. The in-plane spintransfer torque from the in-plane polarization layer 750 thereby assiststhe perpendicular spin transfer torque from the reference layer 732 inswitching the magnetization direction of the free layer. Thus, devicesusing a STNO 770, a metallic bit line 760, and a plurality of OST-MTJstructures 730′ (each comprising an in-plane polarizer 750, and aperpendicular MTJ) can enhance the efficiency of switching the magneticdirection of the free magnetic layer 736 of the selected OST-MTJ 730″during the writing process.

In particular, the structures described herein utilize a STNO that hasbeen designed to produce oscillations in the current value of theswitching current that are synchronized with the predeterminedprecession frequency of the free magnetic layer 736 during the writingprocess for a selected OST-MTJ stack. As described above, this systemprovides a net in-plane spin transfer torque throughout the wholeprecession cycle and therefore significantly enhances the free layerswitching process in both switching directions, which will result infaster write times and lower switching threshold currents.

An alternative embodiment is shown in FIG. 14. In this embodiment,magnetic device 800 has had its STNO stack 860 inverted with respect tothe embodiment shown in FIG. 12. Similarly, magnetic device 800 has hadeach OST-MTJ stack 830′ inverted with respect to the embodiment shown inFIG. 12. In particular, each OST-MTJ stack 830′ includes a seed layer810. In-plane polarization magnetic layer 820 is placed over seed layer810. In-plane polarization magnetic layer 820 has the same properties,construction and characteristics as in-plane polarization magnetic layer750, discussed above. Nonmagnetic spacer 830 is placed over in-planepolarization magnetic layer 820 and has the same properties,construction and characteristics as nonmagnetic spacer 740, discussedabove. MTJ 840 is generally constructed of free layer 842 (which isplaced over nonmagnetic spacer 830) and reference layer 846. Free layer842 and reference layer 846 are spatially separated from each other bytunneling barrier layer 844, which is made of an insulating material.Reference magnetic layer 846 and free magnetic layer 842 have the sameproperties, construction and characteristics as reference magnetic layer732 and free magnetic layer 736, respectively, discussed above. MTJ 840is placed over nonmagnetic spacer 830. Metallic bit line 850 is placedover each OST-MTJ structure (i.e., placed over the plurality ofreference layers 846). Metallic bit line 850 has the same properties,construction and characteristics as metallic bit line 760, discussedabove. Perpendicular spin torque oscillator layer 862 is placed overmetallic bit line 850. Perpendicular spin torque oscillator layer 862has the same properties, construction and characteristics asperpendicular spin torque oscillator layer 776, discussed above.Nonmagnetic spin torque oscillator barrier layer 864 is placed overperpendicular torque oscillator layer 862 and has the same properties,construction and characteristics as non-magnetic spin torque oscillatorbarrier layer 874, discussed above. In-plane spin torque oscillatorlayer 866 is placed over nonmagnetic spin torque oscillator barrierlayer 864. In-plane spin torque oscillator layer 866 has the sameproperties, construction and characteristics as in-plane spin torqueoscillator layer 772, discussed above. Finally, capping layer 870 isplaced over STNO 860. The programming voltage can be provided by avoltage source 885. OST-MTJ stacks can be selected for writing, forexample, using transistors 805. Other than the ordering of the layers,magnetic device 800 operates in the same manner as described withrespect to the embodiment shown in FIG. 7a . Thus, just as shown in FIG.7a , the in-plane spin transfer torque 610 provides a net benefit ofenhancing the efficiency of the switching throughout the entireprecession cycle of free layer 862.

All of the layers of devices 700 and 800 illustrated in FIGS. 8 and 12can be formed by a thin film sputter deposition system as would beappreciated by one skilled in the art. The thin film sputter depositionsystem can include the necessary physical vapor deposition (PVD)chambers, each having one or more targets, an oxidation chamber and asputter etching chamber. Typically, the sputter deposition processinvolves a sputter gas (e.g., oxygen, argon, or the like) with anultra-high vacuum and the targets can be made of the metal or metalalloys to be deposited on the substrate. Thus, when the presentspecification states that a layer is placed over another layer, suchlayer could have been deposited using such a system. Other methods canbe used as well. It should be appreciated that the remaining stepsnecessary to manufacture MTJ stack 700 and 800 are well-known to thoseskilled in the art and will not be described in detail herein so as notto unnecessarily obscure aspects of the disclosure herein.

It should be appreciated to one skilled in the art that a plurality ofMTJ structures 700 and 800 can be manufactured and provided asrespective bit cells of an STT-MRAM device. In other words, each MTJstack 700 and 800 can be implemented as a bit cell for a memory arrayhaving a plurality of bit cells.

The above description and drawings are only to be consideredillustrative of specific embodiments, which achieve the features andadvantages described herein. Modifications and substitutions to specificprocess conditions can be made. Accordingly, the embodiments in thispatent document are not considered as being limited by the foregoingdescription and drawings.

What is claimed is:
 1. A magnetic device comprising: a plurality oforthogonal spin transfer magnetic tunnel junctions (OST-MTJs) in a firstplane, each OST-MTJ comprising an in-plane polarization magnetic layer,a non-magnetic spacer, a reference magnetic layer, a non-magnetic tunnelbarrier layer and a free magnetic layer; the in-plane polarizationmagnetic layer separated from the free magnetic layer by thenon-magnetic spacer, the free magnetic layer and the reference magneticlayer separated by the non-magnetic tunnel barrier layer; the in-planepolarization magnetic layer having a magnetization vector that isparallel to the first plane; the reference magnetic layer having amagnetization vector that is perpendicular to the first plane and havinga fixed magnetization direction, the free magnetic layer having amagnetization vector that is perpendicular to the first plane and havinga magnetization direction that can switch from a first magnetizationdirection to a second magnetization direction and from the secondmagnetization direction to the first magnetization direction, with aswitching process that involves precessions at a precession radiusaround an axis perpendicular to the first plane, the magnetizationvector of the free magnetic layer having a predetermined precessionfrequency; a metallic bit line in a second plane and coupled to theplurality of OST-MTJs; and a spin torque nano oscillator (STNO) in athird plane and coupled to the metallic bit line, the spin torque nanooscillator comprising an in-plane spin torque oscillator layer, anon-magnetic spin torque oscillator barrier layer, and a perpendicularspin torque oscillator layer, the in-plane spin torque oscillator layerand the perpendicular spin torque oscillator layer separated by thenon-magnetic spin torque oscillator barrier layer, the in-plane spintorque oscillator layer having a magnetization vector that precessesaround an in-plane anisotropy axis or precesses in the third plane uponapplication of a programming voltage pulse, the perpendicular spintorque oscillator layer having a magnetization vector that precessesaround an out-of-plane anisotropy axis upon application of theprogramming voltage pulse; wherein application of the programmingvoltage pulse to the magnetic device results in a switching currentpulse, the switching current pulse alternating between a maximum currentvalue and a minimum current value at a first frequency; whereinapplication of the switching current pulse to the in-plane polarizationmagnetic layer, the non-magnetic spacer, the reference magnetic layer,the non-magnetic tunnel barrier layer and the free magnetic layerresults in a spin-polarized current having spin-polarized electrons, thespin-polarized current alternating between a maximum spin-current valueand a minimum spin-current value at the first frequency; wherein thespin-polarized electrons exert a spin transfer torque on the magneticvector of the free magnetic layer, the spin transfer torque alternatingbetween a maximum magnitude and a minimum magnitude at the firstfrequency; wherein the first frequency is synchronized with thepredetermined precession frequency of the free magnetic layer, therebycausing the spin transfer torque to be at the maximum magnitude when thespin transfer torque increases the precession radius of themagnetization vector of the free magnetic layer, and at the minimummagnitude when the spin transfer torque decreases the precession radiusof the magnetic vector of the free magnetic layer, thereby improving theswitching process of the free magnetic layer from the firstmagnetization direction to the second magnetization direction and fromthe second magnetization direction to the first magnetization direction.2. The magnetic device of claim 1, wherein a difference in frequencybetween the first frequency and the predetermined precession frequencyof the free magnetic layer is less than twenty percent of thepredetermined precession frequency of the free magnetic layer.
 3. Themagnetic device of claim 1, wherein a difference in frequency betweenthe first frequency and the predetermined precession frequency of thefree magnetic layer is less than ten percent of the predeterminedprecession frequency of the free magnetic layer.
 4. The magnetic deviceof claim 1, wherein a difference in frequency between the firstfrequency and the predetermined precession frequency of the freemagnetic layer is less than five percent of the predetermined precessionfrequency of the free magnetic layer.
 5. The magnetic device of claim 1,wherein a difference in frequency between the first frequency and thepredetermined precession frequency of the free magnetic layer is lessthan two percent of the predetermined precession frequency of the freemagnetic layer.
 6. The magnetic device of claim 1, wherein the metallicbit line comprises Ruthenium or Rhodium.
 7. The magnetic device of claim1, wherein the metallic bit line comprises a layer of Ruthenium, thelayer of Ruthenium being between 2 and 10 angstroms thick.
 8. Themagnetic device of claim 1, wherein the magnetization vector of thein-plane spin torque oscillator layer and the magnetization vector of atleast one in-plane polarization magnetic layer are magnetically coupled.9. The magnetic device of claim 1, wherein the programming voltage pulsecomprises a direct voltage.
 10. The magnetic device of claim 1, whereinthe reference magnetic layer comprises CoFeB, the non-magnetic tunnelbarrier layer comprises MgO, the free magnetic layer comprises CoFeB,the non-magnetic spacer comprises MgO, and the in-plane polarizationmagnetic layer comprises CoFeB.
 11. A magnetic device comprising: aplurality of orthogonal spin transfer magnetic tunnel junctions(OST-MTJs) in a first plane, each OST-MTJ comprising a referencemagnetic layer, a non-magnetic tunnel barrier layer, a free magneticlayer, a non-magnetic spacer, and an in-plane polarization magneticlayer; the non-magnetic tunnel barrier layer disposed over the referencemagnetic layer, the reference magnetic layer having a magnetizationvector that is perpendicular to the first plane and having a fixedmagnetization direction; the free magnetic layer disposed over thenon-magnetic tunnel barrier layer, the free magnetic layer having amagnetization vector that is perpendicular to the first plane and havinga magnetization direction that can switch from a first magnetizationdirection to a second magnetization direction and from the secondmagnetization direction to the first magnetization direction, with aswitching process that involves precessions at a precession radiusaround an axis perpendicular to the first plane, the magnetizationvector of the free magnetic layer having a predetermined precessionfrequency; the non-magnetic spacer disposed over the free magneticlayer; the in-plane polarization magnetic layer disposed over thenon-magnetic spacer, the in-plane polarization magnetic layer having amagnetization vector that is parallel to the first plane; a metallic bitline in a second plane and disposed over the plurality of OST-MTJs; anda spin torque nano oscillator (STNO) in a third plane and coupled to themetallic bit line, the spin torque nano oscillator comprising anin-plane spin torque oscillator layer, a non-magnetic spin torqueoscillator barrier layer, and a perpendicular spin torque oscillatorlayer, the in-plane spin torque oscillator layer and the perpendicularspin torque oscillator layer separated by the non-magnetic spin torqueoscillator barrier layer, the in-plane spin torque oscillator layerhaving a magnetization vector that precesses around an in-planeanisotropy axis or precesses in the third plane upon application of aprogramming voltage pulse, the perpendicular spin torque oscillatorlayer having a magnetization vector that precesses around anout-of-plane anisotropy axis upon application of the programming voltagepulse; wherein application of the programming voltage pulse to themagnetic device results in a switching current pulse, the switchingcurrent pulse alternating between a maximum current value and a minimumcurrent value at a first frequency; wherein application of the switchingcurrent pulse to the in-plane polarization magnetic layer, thenon-magnetic spacer, and the MTJ results in a spin-polarized currenthaving spin-polarized electrons, the spin-polarized current alternatingbetween a maximum spin-current value and a minimum spin-current value atthe first frequency; wherein the spin-polarized electrons exert a spintransfer torque on the magnetic vector of the free magnetic layer, thespin transfer torque alternating between a maximum magnitude and aminimum magnitude at the first frequency; wherein the first frequency issynchronized with the predetermined precession frequency of the freemagnetic layer, thereby causing the spin transfer torque to be at themaximum magnitude when the spin transfer torque increases the precessionradius of the magnetization vector of the free magnetic layer, and atthe minimum magnitude when the spin transfer torque decreases theprecession radius of the magnetic vector of the free magnetic layer,thereby improving the switching process of the free magnetic layer fromthe first magnetization direction to the second magnetization directionand from the second magnetization direction to the first magnetizationdirection.
 12. The magnetic device of claim 11, wherein a difference infrequency between the first frequency and the predetermined precessionfrequency of the free magnetic layer is less than twenty percent of thepredetermined precession frequency of the free magnetic layer.
 13. Themagnetic device of claim 11, wherein a difference in frequency betweenthe first frequency and the predetermined precession frequency of thefree magnetic layer is less than ten percent of the predeterminedprecession frequency of the free magnetic layer.
 14. The magnetic deviceof claim 11, wherein a difference in frequency between the firstfrequency and the predetermined precession frequency of the freemagnetic layer is less than five percent of the predetermined precessionfrequency of the free magnetic layer.
 15. The magnetic device of claim11, wherein a difference in frequency between the first frequency andthe predetermined precession frequency of the free magnetic layer isless than two percent of the predetermined precession frequency of thefree magnetic layer.
 16. The magnetic device of claim 11, wherein themetallic bit line comprises Ruthenium or Rhodium.
 17. The magneticdevice of claim 11, wherein the metallic bit line comprises a layer ofRuthenium, the layer of Ruthenium being between 2 and 10 angstromsthick.
 18. The magnetic device of claim 11, wherein the magnetizationvector of the in-plane spin torque oscillator layer and themagnetization vector of at least one polarization magnetic layer aremagnetically coupled.
 19. The magnetic device of claim 11, wherein theprogramming voltage pulse comprises a direct voltage.
 20. The magneticdevice of claim 11, wherein the reference magnetic layer comprisesCoFeB, the non-magnetic tunnel barrier layer comprises MgO, the freemagnetic layer comprises CoFeB, the non-magnetic spacer comprises MgO,and the in-plane polarization magnetic layer comprises CoFeB.
 21. Amagnetic device comprising: a plurality of reference magnetic layers ina first plane, each reference magnetic layer having a magnetizationvector that is perpendicular to the first plane and having a fixedmagnetization direction; a plurality of non-magnetic tunnel barrierlayers in a second plane, each non-magnetic tunnel barrier layerdisposed over one reference magnetic layer; a plurality of free magneticlayers in a third plane, each free magnetic layer disposed over onenon-magnetic tunnel barrier layer, each free magnetic layer having amagnetization direction that can switch from a first magnetizationdirection to a second magnetization direction and from the secondmagnetization direction to the first magnetization direction, with aswitching process that involves precessions at a precession radiusaround an axis perpendicular to the third plane, the magnetizationvector of the free magnetic layer having a predetermined precessionfrequency, the plurality of reference magnetic layers, the plurality ofnon-magnetic tunnel barrier layers and the plurality of free magneticlayers forming a plurality of magnetic tunnel junctions; a plurality ofnon-magnetic spacers in a fourth plane, each non-magnetic spacerdisposed over one free magnetic layer; a plurality of in-planepolarization magnetic layers in a fifth plane, each in-planepolarization magnetic layer disposed over one non-magnetic spacer, thein-plane polarization magnetic layer having a magnetization vector thatis parallel to the fifth plane; a metallic bit line in a sixth plane anddisposed over the plurality of in-plane polarization magnetic layers; anin-plane spin torque oscillator layer in a seventh plane and disposedover the metallic bit line, the in-plane spin torque oscillator layerhaving a magnetization vector that precesses around an in-planeanisotropy axis upon application of a programming voltage pulse; anon-magnetic spin torque oscillator barrier layer in an eighth plane anddisposed over the in-plane spin torque oscillator layer; and aperpendicular spin torque oscillator layer in a ninth plane and disposedover the non-magnetic spin torque oscillator barrier layer, theperpendicular spin torque oscillator layer having a magnetization vectorthat precesses around an out-of-plane anisotropy axis upon applicationof the programming voltage pulse; wherein application of the programmingvoltage pulse to the magnetic device results in a switching voltageacross the in-plane polarization magnetic layer, the non-magnetic spacerand the MTJ, the switching voltage oscillating between a maximum voltagevalue and a minimum voltage value at a first frequency; wherein thefirst frequency is synchronized with the predetermined precessionfrequency of the free magnetic layer, thereby enhancing the efficiencyof the switching process from the first magnetization direction to thesecond magnetization direction and from the second magnetizationdirection to the first magnetization direction.
 22. The magnetic deviceof claim 21, wherein application of the switching voltage across thein-plane polarization magnetic layer, the non-magnetic spacer and theMTJ generates a spin-polarized current having spin-polarized electrons,the spin-polarized current alternating between a maximum spin-currentvalue and a minimum spin-current value at the first frequency, thespin-polarized electrons exerting a spin transfer torque on the magneticvector of the free magnetic layer.
 23. The magnetic device of claim 21,wherein the first frequency is synchronized with the predeterminedprecession frequency of the free magnetic layer, thereby causing thespin transfer torque to be at the maximum magnitude when the spintransfer torque increases the precession radius of the magnetizationvector of the free magnetic layer, and at the minimum magnitude when thespin transfer torque decreases the precession radius of the magneticvector of the free magnetic layer.
 24. The magnetic device of claim 21,wherein a difference in frequency between the first frequency and thepredetermined precession frequency of the free magnetic layer is lessthan twenty percent of the predetermined precession frequency of thefree magnetic layer.
 25. The magnetic device of claim 21, wherein adifference in frequency between the first frequency and thepredetermined precession frequency of the free magnetic layer is lessthan ten percent of the predetermined precession frequency of the freemagnetic layer.
 26. The magnetic device of claim 21, wherein adifference in frequency between the first frequency and thepredetermined precession frequency of the free magnetic layer is lessthan five percent of the predetermined precession frequency of the freemagnetic layer.
 27. The magnetic device of claim 21, wherein adifference in frequency between the first frequency and thepredetermined precession frequency of the free magnetic layer is lessthan two percent of the predetermined precession frequency of the freemagnetic layer.
 28. The magnetic device of claim 21, wherein themetallic spacer comprises Ruthenium or Rhodium.
 29. The magnetic deviceof claim 21, wherein the metallic spacer comprises a layer of Ruthenium,the layer of Ruthenium being between 2 and 10 angstroms thick.
 30. Themagnetic device of claim 21, wherein the magnetization vector of thein-plane spin torque oscillator layer and the magnetization vector ofthe spin current magnetic layer are magnetically coupled.
 31. Themagnetic device of claim 21, wherein the magnetization vector of thepolarization magnetic layer is fixed.
 32. The magnetic device of claim21, wherein the programming voltage pulse comprises a direct voltage.33. The magnetic device of claim 21, wherein the reference magneticlayer comprises CoFeB, the non-magnetic tunnel barrier layer comprisesMgO, the free magnetic layer comprises CoFeB, the non-magnetic spacercomprises MgO, and the in-plane polarization magnetic layer comprisesCoFeB.
 34. The magnetic device of claim 21, wherein the in-plane spintorque oscillator layer comprises CoFeB and the perpendicular spintorque oscillator layer comprises CoFeB.